The present invention relates to a pulse shaper and in particular also to a laser with such pulse shaper.
The use of a pulse shaper typically is provided for ultrashort light pulses with a wide frequency spectrum. The objective of a pulse shaper is to manipulate individual spectral components of the light pulse with respect to the others in a temporal, spatial or polarization-dependent manner.
There are known devices which allow to influence wavelengths separately (e.g. LCD mask, AOM). The majority of pulse shapers utilizes a dispersive optical element by means of which the wavelengths are split up and thus made accessible. This involves the disadvantage that the total energy of the light pulse is reduced by reflection losses at the dispersive optical element.
Due to the dispersive splitting of the wavelengths in different directions in space, a temporal shaping and compression of the light pulse can be effected along different travel paths for different wavelengths. This will necessarily result in a change of the beam direction. Moreover, the beam path required in addition for shaping the light pulse is long and typically lies in the visible spectral range between 30 and 300 cm. In the near infrared and middle infrared spectral range, this length even increases. This has a negative influence on the stability of an optical assembly, since the pointing error increases with a longer beam path.
Furthermore, it is difficult in the middle infrared spectral range (between 2500 and 30000 mm) to adjust pulse shapers in the resonator of an IR laser, since the dispersed light beam cannot be made visible. As a result and due to the fact that the optical materials in part are not transparent for visible light in the infrared spectral range, pulse shapers are hardly used in the middle infrared spectral range.
An alternative is the use of an AOM (acousto-optical modulator), which provides for shaping amplitude and phase of the infrared light pulse. However, there are limitations for the diameter of the light beam and for the intensity, which can be very high in ultrashort pulses. In addition, AOMs are very expensive.
Furthermore, it is known from C. J. Fecko et al., Optics Communications, 241 (2004), 521-528, to shape the infrared light pulse in the middle infrared spectral range in a simple manner by means of plane-parallel plates of different materials. Here, the GVD (group velocity dispersion) of different materials in the infrared spectral range is utilized to impart an (almost) linear positive or negative chirp to the light pulse.
Disadvantageously, the plane-parallel plates cannot continuously be varied in their optical thickness in a simple manner, without the beam direction being changed (e.g. when tilting the plates). Another disadvantage consists in that materials which are transparent in the middle infrared spectral range regularly have a high refractive index for this spectral range, so that even when tilting a plane-parallel plate, the light beam would be refracted with respect to the perpendicular to such an extent that the change in length of the optical path would only be insufficient. The resulting necessary thicknesses and/or tilt angles on the one hand would lead to an undesirably high absorption inside the plate and on the other hand to an undesirably high reflection due to the resulting angles of incidence.
In addition, the use of a plurality of plane-parallel plates involves higher reflection losses, which reduces the total intensity of the light pulse.